Non-lithographic photo-induced patterning of polymers from liquid crystal solvents with spatially modulated director fields

ABSTRACT

A liquid crystal device comprises a first and second cell wall structure; at least one liquid crystal material disposed within a space between the first and second cell wall structures; and polymer micro-structures, wherein the micro-structures are formed by polymerizing a prepolymer, and wherein said micro-structures have a shape and spatial location determined by said liquid crystal material. Permanent polymer micro-structures are formed from a liquid crystal with a non-uniform spatially modulated director field. The polymer structures have the shape and spatial location dictated by the non-uniform director field of the liquid crystal. The micro-structures are a backbone that restores the liquid crystal director field that existed during the polymerization process even when other factors, such as electric field, temperature, or surface anchoring, do not favor this restoration. The polymer micro-structures can be used in optical devices, such as diffraction gratings and deflecting and beam steering devices, and in micro-mechanical and micro-fluidic devices.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 09/966,294, whichwas filed on Sep. 27, 2001, now U.S. Pat. No. 6,897,915 and claims thebenefit of U.S. Provisional Application No. 60/235,756, which was filedon Sep. 27, 2000.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has a paid-up license in this invention andmay have the right in limited circumstances to require the patent ownerto license others on reasonable terms as provided for by the terms ofGrant DMR 89-20147, awarded by the National Science Foundation, andGrant F49620-96-1-0449, awarded by the Air Force Office of ScientificResearch.

BACKGROUND OF THE INVENTION

The present invention relates to polymer-liquid crystal mixtures, andmore specifically to forming of polymer architectures such as polymermicro-walls using polymerization in the liquid crystalline matrix withspatially distorted director fields.

The electro-optic properties of polymer-liquid crystal mixtures havemade them useful as elements of various optical devices. Depending onthe relative polymer concentration in the mixture, different types ofdevices have been demonstrated. When the concentration of polymersignificantly exceeds that of the liquid crystal (LC), a polymerdispersed liquid crystal (PDLC) type of device is realized, where the LCis suspended in the form of small droplets in a surrounding polymermatrix. Light, heat, or solvent evaporation can be utilized to inducethe desired phase separation. Light scattering characteristics of thedevice are dependent upon the orientation of molecules within the LC. Ina second type of device, the concentration of polymer is significantlylower than that of the LC. In this case, a polymer stabilized type ofdevice is realized. The LC is mixed with a pre-polymer (monomer), whichis then polymerized. Polymerization of the monomer creates a polymernetwork which stabilizes the LC structure, enabling, e.g., creation ofbistable LC displays.

When concentrations of the polymer and the LC are roughly of the sameorder of magnitude, it is possible, by implementing a holographictwo-beam recording technique, to form an inhomogeneous periodicmorphology which consists of a cross-linked polymer network withembedded LC droplets. This is described by Pogue et al. in SPIE, 3475,pp. 2-11 (1998). Large droplet size contributes significantly tounwanted light-scattering, and it is therefore desirable to reduce thesize of the droplets as much as possible. As shown by Kim et al., Appl.Phys. Lett. 72(18), pp. 2252-2253 (1998), it is possible todirectionally phase-separate the LC and polymer components in ahigh-voltage electric field using lithographic patterning of electrodeson a confining substrate. A difference in dielectric permittivity of thecomponents plays a role in the phase separation.

A technique for the design of switchable diffractive elements with amemorized structure is needed. Shorter switching times and lesslight-scattering are desired. Reliable techniques for producing polymerarchitectures such as walls at micron scales would be advantageous forstabilizing the LC structure, reducing the light scattering andswitching times. The polymer architectures can be used by themselves (bywashing the LC out of the sample, e.g., in micro-mechanical and inmicro-optical devices).

BRIEF SUMMARY OF THE INVENTION

The first aspect of the present invention is to provide a liquid crystaldevice having a memorized structure, wherein the structure comprisespolymer micro-structures such as micro-walls formed by using a spatiallymodulated director field.

Another aspect of the present invention is to provide a method formanufacturing the liquid crystal device set forth above, wherein theintrinsic mesogenic properties of the liquid crystal are employed todetermine the memorized structure of the polymer. This method enablesreliable micron-scale patterning of polymer architectures.

Yet another aspect of the present invention is to provide a techniquefor forming polymer micro-structures such as micro-walls. Themicro-structures form at locations where there are variations in thedirector field of the liquid crystal.

The foregoing and other aspects of the present invention, which shallbecome apparent as the detailed description proceeds, are achieved by aliquid crystal device comprising a pair of opposed substrates having agap therebetween; a liquid crystal material disposed in said gap andpolymer micro-structures formed between said substrates, wherein themicro-structures are formed by polymerizing a prepolymer, and whereinsaid micro-structures have a shape and spatial location determined bysaid liquid crystal material.

Other aspects of the present invention are attained by a method forfabricating a liquid crystal device having polymer micro-structures, themethod comprising the steps of preparing a mixture comprising a liquidcrystal material and a prepolymer; providing a first and second cellwall structure, said first and second cell wall structure optionallyhaving electrodes disposed on facing sides of said first and second cellwall structures, and optionally having an alignment layer disposed on atleast one of said electrodes; disposing said mixture into a spacebetween the first and second cell wall structures; causing said liquidcrystal material to assume a predetermined orientation with anon-uniform spatially distorted director field; and exposing saidmixture to conditions which cause polymerization of the prepolymer andformation of polymer micro-structures between the cell walls.

The present invention also provides a method for forming polymermicro-structures, the method comprising the steps of preparing a mixturecomprising a liquid crystal material and a prepolymer; providing a firstand second cell wall structure; disposing said mixture into a spacebetween the first and second cell wall structures; causing said liquidcrystal material to assume a predetermined orientation with anon-uniform spatially distorted director; and exposing the mixture toconditions which cause polymerization of the prepolymer and formation ofpolymer micro-structures between the cell walls.

These and other aspects of the present invention, as well as theadvantages thereof over existing prior art forms, which will becomeapparent from the description to follow, are accomplished by theimprovements hereinafter described and claimed.

As used herein, the term “director” refers to the average orientation ofthe molecules of a liquid crystal material. The phrase “spatiallymodulated director fields” refers to changes in the orientation of thedirector from one point to another in a liquid crystal sample. Anon-uniform spatially modulated director field refers to a directorfield having variations in the rate of director changes at differentpositions within a liquid crystal sample.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1( a) is a microphotograph of a liquid crystal cell with an appliedvoltage V>2.5 Volts, showing chains of polymer particles which formpolymer micro-structures such as micro-walls.

FIG. 1( b) is a microphotograph of a liquid crystal cell with an appliedvoltage V<2.5 Volts, showing texture growth on top of the polymermicro-structures.

FIG. 2( a) is a microphotograph of a liquid crystal cell with an appliedvoltage V=2.6 Volts (f=1 kHz), showing polymer micro-walls in planargeometry.

FIG. 2( b) is a microphotograph of a liquid crystal cell with an appliedvoltage V=2.6 Volts (f=1 kHz), showing polymer micro-walls in planargeometry, under monochromatic light.

FIG. 3( a) is a schematic representation of ‘Bloch wall’ nematicdirector field used to produce polymer micro-wall, the lengths of nailsare proportional to projections of molecules on xz-plane, heads arepointed inside the plane, the tips are pointed outwards.

FIG. 3( b) is a scanning electron microscope (SEM) image of a polymermicro-wall formed inside the director field shown on FIG. 3( a).

FIG. 4 is a schematic representation of a device used to implement themethod according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that permanent polymer micro-structures, such asmicro-walls, cart be created on the sub-micron scale using the intrinsicmesogenic properties of the LC component in a polymer-LC mixture. Themicro-structures self-assemble by exposing a texture, formed by aspatially distorted LC mixed with a pre-polymer, to conditions whichcause polymerization of the prepolymer. Suitable LC materials includenematic, cholosteric, smectic and columnar LC materials. The amount ofthe pre-polymer added to the LC is between about 0.1 percent and about50 percent, such that the mixture preserves its mesogenic (eithernematic, cholesteric smectic or columnar) properties at a predeterminedtemperature. A distorted arrangement of LC molecules enables the designand implementation of various molecular architectures of the polymercomponent.

Polymerization-inducing conditions cause phase separation of componentsin a LC-polymer mixture. The polymerization-inducing conditions may beselected from a wide variety of conditions known in the art according tothe requirements of a particular application. In one embodiment, forexample, the prepolymer is a UV-curable prepolymer. Suitable UV-curablepre-polymer materials include, but are not limited to, monomers whosepolymerization is of a thiolene-photoinitiated step-growth type, such asNOA-65 (available from Norland Products, Inc.). In such a case, thepolymerization-inducing conditions will include exposure of the liquidcrystal material/prepolymer mixture to ultraviolet radiation. In anotherembodiment, the prepolymer is a heat-curable prepolymer and thepolymerization-inducing conditions include exposure of the liquidcrystal material/prepolymer mixture to a temperature elevated above roomtemperature, i.e. above about 20° C.

The phase separation in the presence of a spatially modulated LCdirector field results in formation of polymer micro-structures in theLC cell. While not wishing to condition patentability on any particulartheory, there are at least three possible mechanisms by which themicro-structures form: (1) Since the liquid crystal is an elasticmedium, the phase separating component such as polymer (or othermaterial) might be forced to accumulate in the sites with the highestenergy of director distortions; (2) polymer density might vary becauseof spatial changes of the angle between polarization director of lightused to cause phase separation and to cure the prepolymer, and thedirector; (3) variations in intensity of light that is used to cure theprepolymer, caused by “lens” effects in the distorted birefringentliquid crystal. In all these cases, phase separation and polymerizationfrom a non-uniformly distorted liquid crystal would lead to a spatialvariation in the density of the phase separating component (polymer), asdictated by the spatial variation of the director field. Using thistechnique, it is possible to make micro-structures between thesubstrates of a cell, e.g., periodic polymer-LC diffractive gratings.

Referring now to the drawings, and in particular, to FIG. 4, it can beseen that a device according to the present invention is designatedgenerally by the numeral 10. The device 10 includes a pair of opposed,optically clear substrates 12, which may be glass, plastic or othermaterial commonly known in the art. The substrates are arranged suchthat a cell gap 14 exists therebetween. The cell gap may be formed withspherical spacers or rods. An electrode 16 may be provided on the inside(facing) surfaces of each of the substrates 12. In a preferredembodiment, each electrode 16 comprises indium-tin oxide.

An alignment layer 18 may be provided adjacent to electrodes 16 in orderto control the orientation of the material enclosed between thesubstrates 12. One or both of the substrates 12 may be treated with anappropriate alignment layer in order to obtain the desired opticalperformance of the liquid crystal device 10. The alignment layer maycomprise polymers such as polyimlides, deposited layers of silicon oxideand surfactants. In one embodiment, the alignment layer comprises apolyimide. Rubbing techniques, photoaligning techniques, coating withsurfactants, and other techniques for preparing alignment layers areknown in the art. In one particular embodiment, the alignment layer isformed by JALS-204 (Japan Synthetic Rubber Co., Inc.).

The director field of a LC device indicates the direction of preferredorientation. Proper preparation of the alignment layer can producealignment of the director field in almost any direction. In oneembodiment, the alignment layer produces a homogeneous planar geometryof the director field, which corresponds to alignment of the directorfield parallel to the substrate surface. In another embodiment, thealignment layer produces a homeotropic geometry of the director field,which corresponds to alignment of the director field perpendicular tothe substrate surface. In still another embodiment, the alignment layerproduces a homogeneous tilted geometry of the director field. In yetanother embodiment, the alignment layer produces a patterned geometry ofthe director field with different alignment properties at differentregions of the cell.

A mixture of a liquid crystal material and a pre-polymer is disposed inthe cell gap 14. The liquid crystal has a non-uniform spatiallydistorted director. The non-uniform spatial director distortionsdetermine the polymer micro-structure that forms during phase separationand polymerization process. There are numerous ways to control thenon-uniform director field of the liquid crystal and thus to design thedesired polymer micro-structure. One way is to use a cholesteric orchiral nematic liquid crystal. In an ideal cholesteric, the directorforms a helix because of the chiral nature of the mesogenic molecules.In a bounded sample (for example, inside gap 14) the cholestericstructure might acquire additional distortions caused by the type ofsurface alignment set by the alignment layer(s) 18, especially when thecholesteric helicoidal axis is in the plane of the sample (so-called“fingerprint cholesteric structure”). Another way is to apply a voltageacross electrodes 16. In one embodiment, two aligning layers 18 areformed in such a way that the rubbing directions are mutuallyperpendicular. A nematic liquid crystal filled in gap 14 for such a cellwould acquire twist deformations with the director continuously twistingfrom the top substrate to the bottom. There will be left and righttwists between the substrates, with domain boundaries separating thedomains of left and right twist. These domain boundaries serve as theplaces where the polymer accumulates during polymerization. Otherdesigns are possible, in which the director structures are distorted bythe applied electric or magnetic fields. For example, a planarcholesteric layer with helical axis being initially perpendicular to thesubstrates 12, can be distorted by applying an electric voltage to theelectrodes 16; the field reorients the helical axis away from theinitial orientation perpendicular to the bounding plates and thuscreates a non-uniformly distorted director structure, periodic in one ortwo directions in the plane of the liquid crystal cell. In any event,once the desired director structure is achieved, the prepolymerdissolved in the liquid crystal is polymerized by, for example, exposureto radiation from a UV source 20. As the polymer forms, phase separationoccurs. The polymer forms a micro-structure 22 surrounded by the liquidcrystal 24. The spatial shape and position of the micro-structure isdefined by the spatially modulated director field. In one embodiment,the micro-structures are affixed to at least one of the substrates.

A power source 26 may be attached to the electrodes 16 through a switch28. Operation of the switch 28 may be controlled by an appropriatelydesigned electronic drive.

In one embodiment, polymer micro-structures are formed using afingerprint texture of a cholesteric liquid crystal. Cholesteric liquidcrystals are sometimes called chiral-nematic liquid crystals. They areformed by some optically active organic compounds, such as cholesterylchloride, or mixtures of such compounds, or by mixing optically activecompounds with ordinary nematic liquid crystals. A cholesteric has ahelical structure, and the director rotates spatially about an axisperpendicular to itself. The distance for a 360° turn of the director iscommonly referred to as the pitch, which may be of the order of awavelength of light. In the ideal unbounded cholesteric phase comprisingnematic liquid crystals doped with optically active molecules, the pitchis defined by the concentration of the chiral dopant in the nematicmatrix.

The cholesteric material is mixed with a pre-polymer material andconfined between two transparent electrodes with either homogeneousplanar or homeotropic alignment layers. Suitable pre-polymer materialsinclude, but are not limited to, UV-curable monomers whosepolymerization is of a thiolene-photoinitiated step-growth type, such asNOA-65 (available from Norland Products, Inc.).

The fingerprint texture of a cholesteric liquid crystal is the patternformed when the helical axes are parallel to the substrates 12. When ahomogeneous planar alignment layer is utilized, the fingerprint textureis created by applying an electric field to the cholesteric mixture. Theapplied electric field will preferably be from about 0.1 V to about 100V, preferably from about 0.1 V to about 10 V, and will vary dependingupon the values of cell gap 14, confinement ratio d/p, and anisotropy inthe dielectric permittivity of the liquid crystal. In homeotropicgeometry, no field is necessary to sustain a fingerprint texture becauseit exists at zero field. During the “writing process” (the process bywhich the polymer micro-structures are formed), exposure to light(usually, in the UV part of the spectrum) causes polymerization of thepre-polymer with its subsequent phase separation from the cholesteric.The polymer forms micro-structures as it polymerizes on a confiningsubstrate. The micro-structures are permanently affixed, and portray thestructure of the fingerprint texture. The micro-structures are eithercontinuous or resemble chains of polymer particles. Applying a highervoltage to the cholesteric, one can remove the fingerprint texture, butcannot remove the formed polymer micro-structures. In switching thestructure back to the fingerprint state, the polymer structures are abackbone on which the exact original texture that was used in thewriting process restores itself. The stabilization of the fingerprinttexture in the irradiated cell is caused by the change in surfacealignment at the boundaries, namely, by the imprint formed by thepolymer. Micro-structures are created non-lithographically through thepresence of director modulations, their thickness is defined by acharacteristic width of director modulations. The micro-structures canfirst appear in the bulk of the cell or at the confining substrate andgrow as the process of polymerization proceeds. As the polymerstructures grow in size, they adhere to the confining substrates. Incholesteric fingerprint structures, accumulation of the polymer at oneor both substrates is facilitated by the fact that the cholestericdirector distortions are stronger near the surfaces than in the bulk ofthe cell. Surface anchoring sets a particular direction of director atthe substrates and the cholesteric helical twist is usually incompatiblewith such a unidirectional alignment, thus the director is stronglydistorted near the surface to accommodate for both the surface anchoringdirection and the helicoidal twist in the bulk. Usually, themicro-structures are permanently affixed (i.e., they are stable againstgentle mechanical action or against solvents that might dissolve theliquid crystal and wash out from the cell but leave the polymermicro-structures intact), and portray the structure of the fingerprinttexture. In prior art, the structures were created lithographically,their thickness was defined by the size of inter-pixel areas (typically,5-6 microns). With micro-structures made using director modulations,forming micro-structures with thicknesses of 1 micron is simplified.

EXAMPLE 1

In Example 1, polymer micro-structures were formed in cholesteric cellswith a homeotropic geometry. A LC cell was assembled from a pair ofglass plates coated with transparent electro-conductive layers of indiumtin oxide and a lecithin surfactant. A cholesteric mixture of E7 andCB15 (both from EM Industries) with a pitch p of about 6 microns wasmixed with 5.0 percent by weight of the UV-curable optical adhesiveNOA-65. The mixture was introduced into a cell of gap d of about 5.0microns. The mixture was in the cholesteric phase at room temperature.In such a cell fingerprint texture exists at zero applied field. Thecell was exposed to a low power non-polarized UV light of wavelengthλ=366 nanometers for about 10 minutes. Under such conditions,cross-linking of the pre-polymer NOA-65 occurs, with a subsequent phaseseparation of the liquid crystal from a formed polymer. Applying avoltage V of about 2.5 Volts at a frequency f of about 1 kiloHertzbrings the cholesteric into a quasi-homeotropic state and removes thefingerprint texture. The polymer micro-walls are visible through amicroscope, as shown in FIGS. 1( a) and (b). The fingerprint texturerestores itself if the voltage is switched off. FIG. 1( b) shows howseparate fingers of the texture 30 grow on top of the polymermicro-walls 32.

EXAMPLE 2

In Example 2, polymer imprints were formed in cholesteric cells with aplanar geometry. The liquid crystal cell was assembled from a pair ofglass plates coated with transparent electro-conductive layers of indiumtin oxide and unidirectionally rubbed polyimide PI-2555 (available fromDu Pont). The cholesteric/pre-polymer mixture, described above inExample 1, was introduced into a cell of gap d of about 7 microns.

When the flat cell described above was filled with a cholesteric liquidcrystal, the helical axis was normal to the cell plates and there was nodirector modulation in the plane of the cell at zero applied field. Whena voltage was applied, the result was a fingerprint texture withdirector modulations in the plane of the cell. In the initial state, thecell was kept at zero applied field. A modulated cholesteric structureappeared above a threshold voltage V_(th)=2.2 Volts, with ‘stripes’parallel to the rub directions on the substrates. At constant voltageV>V_(th), the cell was exposed to UV light as in Example 1. FIG. 2 ashows the texture in the illuminated cell at an applied voltage of about2.6 Volts at a frequency of about 1 kiloHertz. FIG. 2 b shows thetexture under monochromatic light at an applied voltage of about 2.6Volts at a frequency of about 1 kiloHertz.

EXAMPLE 3

In Example 3, polymer micro-structures in the form of continuousmicro-walls were formed in nematic cells with a homeotropic geometry. ALC cell was assembled from a pair of glass plates coated withpoly(vinyl)cinnamate (Aldrich Chemical Co., Inc) and alignment layersirradiated with unpolarized TV light to produce homeotropic alignment.The nematic E7 (EM Industries) was introduced into a cell of gap d ofabout 2 microns. An inhomogeneous nematic director configuration was inthe form of the ‘Bloch wall’, FIG. 3( a). Such director distortionexists in a nematic sample with its gap, d, less than the anchoringextrapolation length L: d<L=K/W, where K is an average elastic constantof the nematic LC, and where W is the anchoring coefficient of thenematic on the alignment layer. The wall width (˜1.8 micrometers) isdefined by the strength of the in-plane anchoring balancing the nematicelasticity. ‘Bloch wall’ director distortion exists in zero-field. Thecell was exposed to UV-light as in Examples 1, 2. UV-light causedpolymerization of the monomer NOA-65 with its subsequent phaseseparation and polymer micro-wall formation. Polarizing-microscopyobservations and scanning-electron microscopy of the disassembled cell,FIG. 3( b), revealed that the polymer micro-wall formed exactly alongthe nematic distortion ‘Bloch wall’.

It will be appreciated by those skilled in the art that the advantagesof the present invention are numerous. Spatially modulated directorfields can be used to induce patterning of polymers from liquid crystalsolvents. The polymers form micro-structures having a pattern whichfollows the director texture of the liquid crystal. These patternedmicro-structures form a framework for repeatedly and accuratelyrecreating the exact director texture of the liquid crystal. Thistechnique can be used to design switchable diffractive elements with amemorized structure. The diffractive grating can be switched on and offrepeatedly. Because the grating does not need to grow, switching timesare shorter. At the same time, the grating structure is more flexiblethan those of the prior art, where the polymer network was employed tostabilize a cholesteric structure. Greater flexibility allows broaderswitching possibilities and allows having lower values of the switchingvoltage than heretofore possible. The uniform morphology of themicro-structures significantly reduces unwanted light-scattering overthe periodic micro-droplet morphologies of the prior art. Little or noelectric field is required to produce the pattern of polymermicro-structures. No lithographic patterning of electrodes is needed,which allows reliable micron-scale patterning. Utilizing the mesogenicproperties of the liquid crystal component enables precision on theorder of 0.5 microns, a substantial improvement over processes known inthe prior art. The process of the invention can be used in manufacturingoptical elements such as spatial modulators, diffractive gratings, lightdeflectors and beam steeling devices.

1. A liquid crystal device comprising: a pair of opposed substrateshaving a gap therebetween; a liquid crystal material disposed in saidgap; and polymer micro-structures formed between said substrates,wherein the micro-structures are formed by causing the liquid crystalmaterial to assume a predetermined orientation with non-uniformspatially modulated director and thereafter polymerizing a prepolymer,and wherein the micro-structures have a shape and spatial locationdetermined by a director field of said liquid crystal material.
 2. Aliquid crystal device according to claim 1, wherein saidmicro-structures are affixed to said at least one of the substrates. 3.A liquid crystal device according to claim 1, additionally comprising analignment layer disposed on at least one of said substrates.
 4. A liquidcrystal device according to claim 3, wherein said alignment layer isselected from the group consisting of polymers, silicon oxide layers andsurfactants.
 5. A liquid crystal device according to claim 3, whereinsaid alignment layer produces a homogeneous planar geometry of thedirector field.
 6. A liquid crystal device according to claim 3, whereinsaid alignment layer produces a homogeneous tilted geometry of thedirector field.
 7. A liquid crystal device according to claim 3, whereinsaid alignment layer produces a homeotropic geometry of the directorfield.
 8. A liquid crystal device according to claim 3, wherein saidalignment layer produces a patterned geometry of the director field withdifferent alignment properties at different regions of the cell.
 9. Aliquid crystal device according to claim 1, wherein said liquid crystalmaterial is selected from the group consisting of nematic liquid crystalmaterial, cholesteric liquid crystal material, smectic liquid crystalmaterial and columnar liquid crystal material.
 10. A liquid crystaldevice according to claim 1, wherein said prepolymer is selected fromthe group consisting of UV-curable prepolymers and heat-curableprepolymers.